In optical inspection of articles having surfaces of two or more materials, such as the articles mentioned above, a surface of the article is illuminated with light, and the image of the illuminated surface is collected by a light sensor and analyzed for defects. Various types of systems are known for this purpose.
One known type of optical inspection system, particularly useful for inspecting epoxy fiberglass substrates having metal conductors printed thereon, is based on the difference in fluorescence between the substrate and the conductors, to provide high contrast in the image of the two materials. Typical substrate materials fluoresce under appropriate illumination, appearing bright, while the metal pathways which do not fluoresce appear dark in the imaging system. However, this type of optical inspection cannot be used to distinguish between two non-fluorescing materials, such as two metal layers, or to image a thin insulating film on a metal layer which does not fluoresce because of its composition or thinness.
Another type of optical inspection system is based on the differences in reflectivity of the two materials. However, this type of system is not effective where the materials have similar reflectances.
A further type of known optical inspection system is based on a modification of the reflectivity system described above. In this system, the contrast between the two materials in the image is improved responsive to difference in their ability to preserve the polarization state of incident light. This method utilizes a pair of polarizers, one between the source and the object, and the other between the object and the detector. By using the relative orthogonal orientation of the polarizers, it is possible to obtain reasonable contrast between certain combinations of low reflectance materials, such as diffusely-surfaced (i.e. nonspecular) metals or laminates. However, this type of system cannot readily distinguish between two materials having the same or similar characteristics with respect to the preservation of the polarization state of incident light.